The uptake of potassium ions in yeast cells occurs primarily through two specific proteins--a high and a low affinity potassium ion transporter. Rodriguez-Navarro and Ramos, J. Bacteriol, 159:940-945 (1984); Gaber, Styles, and Fink, Mol. Cell Biol. 8:2848-2859 (1988); Ko and Gaber, Genetics 125: 305-312 (1990). Two genes, designated TRK1 and TRK2 (for transporter of K) encode the high and low affinity transporters, respectively. Yeast strains that contain mutations in TRK1 and TRK2 exhibit a growth defect on media with minimal potassium ion (K.sup.+) concentrations. In comparison to wild-type strains, which grow on media with as little as 5 .mu.M potassium ion, transporter mutant strains require media supplemented with 100 mM potassium ion. Two laboratories used such deficient strains to isolate foreign plant genes that complement the growth defect on potassium-deficient media. Anderson, et al., Proc. Natl. Acad. Sci. U.S.A. 89: 3736-3740 (1992); Sentenac, et al., Science 256: 663-666 (1992).
In other species, a number of potassium channels have been cloned by molecular biological techniques. Many of these cloned channels are voltage-dependent potassium ion channels related to the Drosophila shaker channel. Several others are novel potassium ion channels. Hille, Ionic Channels of Excitable Membranes (1992); Strong et al., Mol. Biol. Evol. 10: 221-242 (1993).
One such newly discovered channel is the minK, or I.sub.sk, or IK.sub.s, channel, which has only 129-130 amino acids. Murai et al., Bioch. Biophys. Res. Comm. 161: 176-181 (1989); Swanson et al., Sem. Neurosci. 5: 117-124 (1993). A recent report indicates that residues 41 to 72 define the pore of the protein and are sufficient to form channels in lipid bilayers. Ben-Efraim et al., Biochemistry 32: 2371-2377 (1993). The channel minK displays very slow activation and inactivation kinetics. Philipson and Miller, TIPS 13:8-11 (1992). It is expressed in a number of tissues, including kidney, uterus and heart and in a number of species including humans. Swanson et al., Sem. Neurosci. 5: 117-124 (1993); Freeman and Kass, Biophysical J. 64: 342 (abstract) (1993).
Of considerable interest is the role of minK in the heart. It was known that ventricular cells have a repolarzing current, called the delayed rectifier. Recent evidence suggests that the delayed rectifier has two components: a rapid, rectifying component, I.sub.Kr, and a slow component, I.sub.Ks. Sanguinetti and Jurkiewicz, J. Gen Physiol. 96:195-215 (1990). I.sub.Kr is blocked by such Class III antiarrhythmics as d-sotalol, dofetilide, and clofilium; I.sub.Ks, in contrast, is blocked by clofilium but not by d-sotalol or dofetilide. Honore et al., EMBO J. 10:2805-2811 (1990); Jurkiewicz and Sanguinetti, Circ. Res. 72: 75-83 (1993). When expressed in Xenopus oocytes, minK exhibits kinetics and pharmacology similar to those of I.sub.Ks found in isolated myocytes, including blockage by clofilium. Honore et al., EMBO J. 10:2805-2811 (1991 ); Adelman, personal communication. Immunological detection of minK protein in cells possessing I.sub.Ks has recently been reported. Freeman and Kass, Biophys. J. 64:342 (abstract) (1993). These results suggest that the minK product is the slow component of the delayed rectifier.
At faster heart rates, I.sub.Ks contributes to the shortening of the action potential observed at rapid heart rates. Jurkiewicz and Sanguinetti, Circ. Res. 72: 75-83 (1993). Thus, a specific inhibitor of the minK channel would be an effective antifibrillatory or anti-arrhythmic agent. Activators of the minK channel, on the other hand, may be anti-ischemic agents. A need exists, therefore, for methods of screening for inhibitors and/or activators of the minK channel.